1. Technical Field
The invention relates to a sliding element, in particular a piston ring, having a coating, and to a method for coating a sliding element.
2. Related Art
In many technical applications, sliding elements are in sliding contact with sliding contact surfaces. The combination of a piston ring with a cylinder liner can be seen as a typical application. Provided that the cylinders or cylinder liners are made, for example, of aluminium-silicon alloys, which is the case in particular with Otto engines, DLC (diamond-like carbon) coating systems have proved to be successful as regards wear and friction power. However, the known DLC coating systems are in need of improvement as regards lifetime and their usability in diesel or turbocharged Otto engines with conventionally iron-based cylinder liners. Because of the significantly higher cylinder pressures and in particular in combination with direct injection, an increased proportion of mixed friction occurs. The definitive reason for the lack of suitability of DLC coating systems for such situations is seen as being the conventionally small layer thickness of less than 5 μm.
DE 10 2005 063 123 B3 discloses a DLC coating with a running-in layer. With regard to greater layer thicknesses, there are also known PVD coatings, in particular based on CrN, which have layer thicknesses in the range of from 10 to 30 μm. Although these result in an improved lifetime, the friction power and wear resistance, in particular in conditions of insufficient lubrication, and the scuff resistance are impaired. By contrast, DLC coating systems offer the advantage, owing to their amorphous structure, that they exhibit a large degree of chemical inactivity with metal surfaces and accordingly an extremely low adhesive tendency towards the sliding contact surface.
This is true, for example, of the coating according to DE 10 2008 016 864 B3, which has on the inside a metal-containing amorphous carbon layer and on the outside a metal-free amorphous carbon layer.